The long term goal of this project is to develop strategies to improve treatment and prevent breast cancer. The aromatase inhibitors that we pioneered in earlier phases of this grant are now proving to be effective in the clinic. During the current period, we have made a number of significant observations indicating that breast tumor aromatase expression has an important role in tumor promotion. These findings form the basis for the proposed studies. We have detected significant aromatase expression in tumors of about 60 percent of patients. We now propose to investigate in Specific Aim 1, whether intratumoral aromatase expression, measured by our ICC method in paraffin sections, is correlated with the response of breast cancer patients to aromatase inhibitor treatment. This information may predict the value of inhibitor treatment for individual patients. We have successfully isolated cells from normal human breast tissue and from metastatic breast cancer. In Specific Aim 2, we will determine the mechanisms of aromatase gene regulation in luminal epithelial cells and fibroblasts from normal human breast tissue and in metastatic breast cancer cells. We will identify the promoter regions of the aromatase gene controlling expression of the enzyme. In Specific Aim 3, we will determine whether aromatase inhibitors are more effective in combination with specific estrogen receptor modulator raloxifene than either alone in our intratumoral aromatase model. In addition, we will investigate mechanisms of letrozole antitumor action and development of resistance and the effect of antiestrogens and other aromatase inhibitors on letrozole resistant growth this model. These studies could provide important new leads into the mechanisms of aromatase inhibitor antitumor actions and development of resistance and may ultimately be useful for identifying patients who are likely to be responsive/resistant to treatment.